Conventional call signals transmitted over a subscriber line of a telephone system consist of intermittent oscillations with a frequency below the usual voice band, generally of 20-50 Hz, which actuate an electromagnetic relay. An alternating current capable of energizing such a relay must be of considerable amplitude, resulting in appreciable energy losses along the line.
Recent developments in the telecommunication art have led to the gradual replacement of such electromagnetic ringers by electronic transducers responsive to low-voltage voice-frequency signals, such as encapsulated sound generators or miniature loudspeakers excited by crystal-controlled oscillators. Transducers of this type have an efficiency greatly superior to that of conventional electromechanical devices; they can be energized from an associated central office or exchange by the flow of a small direct current, generally of less than 1 mA, insufficient to initiate the establishment of an operating connection between the central office and the subscriber station.
Thus, a transducer of this type may be actuated by a call signal of a few hundred mV in a range of voice frequencies, e.g. between 300 and 3400 Hz, also used for signaling the free or engaged state of a line. At the central office, therefore, a common oscillator may be used for generating dial-tone or busy signals, transmitted to a subscriber station in its active state, and a call signal transmitted to an idle subscriber station. Such an arrangement also entails a considerable simplification of the subscriber's circuitry.
A changover from the conventional electromagnetic ringing system to the electronic one necessitates an at least temporary coexistence of the two systems so that a subscriber station may receive call signals of either type. The problem arises, therefore, of providing such a station with a receiver responsive to both low-power call signals of relatively high frequency and high-power call signals of relatively low frequency.